1. Field of the Invention
This invention relates to an immersion microscope objective, and more particularly to an immersion microscope objective of the apochromatic class which enables the numerical aperture (NA) to exceed 1.4.
2. Prior Art
Of vital importance for a microscope objective is to allow it to have a high numerical aperture (NA), thereby increasing the resolving power. In this regard, numerous proposals have so far been made in the art. For instance, JP-A 61(1986)-275813 and 4(1992)-311703 disclose microscope objectives which have a numerical aperture of 1.4, are well corrected in terms of chromatic aberration, and are improved in terms of the flatness of image surface as well. Since the immersion oil used has a refractive index of about 1.52, however, it is virtually impossible to achieve a further high numerical aperture, because the axial bundle from an object point on the axis is so diffused that much difficulty is involved in lens design per se. Thus, no objective with NA&gt;1.4 is available.
An objective with NA&gt;1.4 belongs virtually to an unheard-of technology. In recent years, however, microscopes have been used in various ways and differential interference has been used in high-tech fields so that the image can be magnified for video camera observation. The objective of the present invention, as will be explained later, can find some way to the realm of the unknown.
Set out below are objectives which are different in specifications from, but similar in construction to, the microscope objective according to the present invention. JM-B 41(1966)-12378 is not directed to an immersion objective, as can be seen from an NA of 0.8 and a magnification of 65.times., but the third lens unit disclosed is similar in shape to the third lens unit of the invention which is one feature of the present invention. It is here to be understood that the present invention distinguishes over JM-B 41-12378 in that the first lens unit does not include a cemented doublet with the cemented surface being concave on the object side and the second lens unit includes no cemented triplet.
U.S. Pat. Nos. 3,700,311 and 4,373,785 are directed to an immersion objective having an NA of 1.3 and a magnification of 100.times., which is identical with the present invention in terms of the first, fifth and sixth lens units and similar to the present invention in terms of the shape of the third and fourth lens units as well. However, the present invention distinguishes over them in that the second lens unit includes no cemented triplet and the third lens unit has positive refracting power. In this connection, it is to be noted that the third lens unit of the present invention is characterized by having negative refracting power.